Metal Gear(メタルギア,Metaru Gia?) is an overhead military action-adventure game originally released in 1987 by Konami for the MSX2 computer standard in Japan and parts of Europe. A reprogrammed version for the Famicom was released in Japan on December 22, 1987. While the MSX2 version of this game was never released in the U.S., the NES version came to North America in June 1988 (published by Konami's Ultra Games division), followed by Europe and Australia in March 1989.

According to Kojima's account, the NES port was developed by another Konami team at Tokyo without his involvement, who were given the source code from the MSX2 version without the consent of Kojima or anyone who worked on the original MSX2 version. While the NES version was intended to be a port of the MSX2 game, many changes were also made to the game during the porting process. Hideo Kojima has been vocal about his disappointment at the changes made to the game in its NES port.

The biggest change to the game was in the overall level design. Instead of the underwater infiltration from the MSX2 game, the NES version features a different opening sequence showing Solid Snake and three other soldiers (who are never seen nor mentioned after the opening) parachuting into the middle of a jungle, where the player begins (instead of starting directly at the entrance of the first building). The Metal Gear robot before the final boss battle is missing. Other changes includes the lack of the second alert phase, different background music, and the removal of Big Boss' message after the end credits.

Like the English localization of the MSX2 version, the English localization of the NES game contains numerous examples of grammatically-incorrect phrases, such as "Contact missing our Grey Fox", "The truck have started to move!", and "I feel asleep!!" The English localization of the NES version's manual swayed from the game's actual storyline. The main villain (whose identity is not revealed until the end) is mentioned to be "Colonel Vermon CaTaffy", while Snake's commanding officer is renamed "Commander South". Additionally, Solid Snake's back-story is different, with the manual claiming that he participated in the Grenada invasion prior to joining FOXHOUND. It should be noted that these alterations to the plot only appear in the manual, but never in the game itself; moreover, the plot summary in the Japanese Famicom version's manual, along with Solid Snake's character profile, is identical to the MSX2 version's Japanese manual.

The NES version of Metal Gear was the basis of two computer ports that were released in 1990 for MS-DOS and the Commodore 64 in North America and Europe. The PC MS-DOS conversion was programmed by Charles Ernst, while the Commodore 64 conversion was handled by Unlimited Software Inc. The PC MS-DOS version contains many minor changes, such as a faster-depleting health bar. However, the Commodore 64 version is closer to the NES version, with only small musical and visual changes. A third computer conversion for the Amiga is listed on the rear packaging of both computer versions, but was never released. A bonus game disc featuring the Famicom version of Metal Gear was included in the Premium Package edition of Metal Gear Solid: The Twin Snakes released only in Japan.

The player's character is Solid Snake, a rookie member of the fictional special forces group FOXHOUND sent on his first mission. He is assisted via radio by his commanding officer Big Boss, who offers information about mission objectives and items; as well as a local Resistance movement composed of Schneider, who provides the locations of important items; Diane, who provides information on how to defeat enemy bosses; and Jennifer, who assists Snake as an inside agent. Among the prisoners Snake rescues includes Grey Fox (Gray Fox in the later versions), a FOXHOUND agent who was captured during a previous mission; Dr. Pettrovich (Dr. Madnar in later releases), the scientist who designed the Metal Gear weapon; and his daughter Ellen, who was kidnapped along with him.